The invention concerns a filter element for filtering a fluid, comprising a filter bellows of a filter medium folded in a zigzag shape, in particular for use as a round filter of an internal combustion engine, in particular of a motor vehicle, as well as a method and an apparatus for manufacturing such a filter element.
End face folds are the two outer folds on opposite end faces of the filter bellows of a filter element. End face borders are the two free borders of the filter medium of which the filter bellows is formed which extend along the end face folds and delimit them at the end faces of the filter bellows. End face edges of the filter bellows are the two other free borders of the filter bellows which extend between the end face borders and extend in accordance with the folding of the filter bellows. The fold edges are the edges along which the filter medium is folded. In case of an approximately parallelepipedal filter bellows that is commercially known and folded in a zigzag shape, the end face borders and the fold edges generally are straight and extend parallel to each other. Viewing the filter bellows from the side, the end face edges extend in a zigzag shape and perpendicular to the end face borders and the fold edges. Prior to folding the filter medium, the end face edges of the future filter bellows, having an imaginary envelope of a substantially parallelepipedal shape, extend straight and parallel to each other. The imaginary envelope is defined by the end face edges, the neighboring fold edges or end face borders at a clean fluid side of the filter bellows, and the neighboring fold edges or end face borders at a raw fluid side.
In filter bellows of flat filter elements, the filter media are not closed, i.e., the end face folds, like the end face edges, are not connected to each other. In contrast thereto, in filter bellows of round filter elements the filter media are closed, i.e., their end face folds are connected to each other. Filter bellows of flat filter elements can be planar but can also have bends in different directions.
In filter systems in which repeated or cyclic fluctuations of the flow rate of the fluid to be filtered through the filter occur, failure of the filter due to fatigue can present a problem. This occurs in particular in systems with great flow rate changes, i.e., from zero to full flow rate and back to zero, as occurs, for example, in filter systems for piston pumps as they are used in energy facilities. These flow cycles cause corresponding cycles in the differential pressures in the filter which typically result in a breathing bending action of the folded medium of the filter. When the folded medium is comprised of a filter layer and a support and drainage layer, the support and drainage layer can rub back and forth against the filter layer upon bending of the folded medium. Since the support and drainage layer is typically more rough than the filter layer, this rubbing action can cause weak points which may rupture in operation. This type of failure is known as fatigue failure.
EP 0 470 485 A2 discloses a filter system that comprises a folded filter element with longitudinally extending folds with round peaks and a wrap member that is spirally wound about the filter element and is contacting the round peaks of the folds and is provided with openings for enlarging the dirt capacity of the filter element, wherein the total surface area of the openings is smaller than approximately half of the entire surface area that is defined by the round peaks of the folds. Moreover, the filter element comprises a folded composite with a first and a second extruded polymeric mesh and a filter layer which is arranged between them.